1. Field
Implementations of the present invention relate generally to the passive detection, classification and identification of combustion events (e.g. muzzle flash from a gun, the flash from an explosion, or the plume of a rocket), more particularly to threat warning systems used to passively monitor potential threats. Each of the various implementations further involves a method for autonomously monitoring the environment and detecting combustion events, a method for capturing and analyzing the event signature, and a method for rendering a determination of the identity of the event in a timely fashion.
2. Brief Description of an Illustrative Environment and Related Art
Passive warning systems autonomously detect and declare threats based on observable quantities intrinsic to the event. These sensors also report the location of the event so that appropriate countermeasures can be initiated. For combustion events, the combustion itself is intrinsic to the event, and the signature from combustion invariably contains strong thermal emission signatures due to the high temperatures that are inherent to burning materials.
Guns, rockets and bombs are all heat engines that use explosive materials to perform mechanical work. The explosive materials undergo a very rapid transformation into more stable materials thereby releasing heat, which is converted to mechanical energy. The process of creating the heat leads to fluorescence of some of the constituents, while the heat itself leads to blackbody radiation from the gasses present in the immediate vicinity of the explosion.
The explosive materials selected for a specific application depend on the application. Low explosives create sub-sonic explosions, while high explosives are capable of super-sonic explosions that result in a detonation shock wave with a powerful blast. Explosives are also classified as primary or secondary based on susceptibility to ignition. Primary explosives are highly susceptible to ignition and are therefore unstable. Secondary explosives are more stable, but more difficult to ignite. Invariably a combination of explosives is used to obtain the appropriate balance of explosive energy and ignitability.
Propellants are used in guns and rockets. Rocket propellants are typically based on a rubber binder, ammonium perchlorate oxidizer and a powdered aluminum fuel. Gun propellants are usually nitrocellulose or nitrocellulose and nitroglycerine. Explosives for bombs fall into three major classes, nitroaromatics (e.g., tri-nitrotoluene or TNT), nitramines (e.g., hexahydro-1,3,5 trinitroazine or RDX), and nitrate esters (e.g., nitrocellulose and nitroglycerine).
Combustion events radiate strongly across a wide band of the electromagnetic spectrum, typically covering the visible, near infrared, short-wave infrared and the mid-wave infrared regions (0.5-5.5 um). The observed signatures depend on the temperature, size, and constituents of the flash when it is visible.
The signatures can have a temporal duration of less than 1/500th second or longer than a minute. The temporal evolution can be an abrupt rise and fall, or it can have multiple maxima intermixed with long regions of constant emission. Often the spectral content varies in time, which is typically observed as a shift in the color temperature in time.
When an event location is known, conventional spectral imaging techniques are adequate for characterizing the spectral signature of the combustion. Spectral imaging is the art of quantifying the spectral and spatial characteristics of a scene within a “field of view.” Optical devices known generally as imaging spectrometers have been developed for measuring and analyzing the spectral content of electromagnetic radiation in various ranges within the spectrum of optical wavelengths. These include, by way of non-limiting example, the ultraviolet; visible; and near, short-wave, mid-wave and long-wave infrared ranges of the electromagnetic spectrum. For purposes of this specification, and the appended claims, all wavelengths of the electromagnetic spectrum are included within the definition of “light,” regardless of visibility with respect to the human eye. In other words, the terms “light,” “electromagnetic energy” and “electromagnetic radiation” are regarded as wholly interchangeable and may be used interchangeably throughout the specification.
Spectral images are typically acquired by scanning the image of a slit across the image of an overall scene, but many hardware configurations that execute alternative imaging modes are available. A spectral image usually consists of a sequence of monochromatic images, wherein each monochromatic image represents the scene as it would appear when viewed over a limited wavelength band and each image in the sequence is centered at a unique wavelength. Accordingly, spectral images are inherently three-dimensional (i.e., they include two spatial dimensions and one spectral dimension) and, therefore, some type of multiplexing is required in order to acquire and display the data in two dimensions.
Three current and emerging multiplexing methods are (1) temporal multiplexing, (2) multiplexing at the image plane and (3) multiplexing at a pupil. Temporal multiplexing is commonly used to acquire image data; however, temporal multiplexing introduces artifacts when the scene is not static. Therefore, most spectral imagers work well for scenes consisting of static objects, but fail to accurately represent scenes including events that evolve rapidly in time (i.e., combustion events).
Since combustion events evolve rapidly in time and their location is unknown, conventional sensors are not able to satisfactorily characterize their signatures. Accordingly, a need exists for a method and apparatus for analyzing and characterizing the spectral signature of combustion events in an environment where their location and timing is unknown.